Children's Attachment Styles

I read over Chapter 10 and primarily focused on children's attachment styles. I found this intriguing to read and insightful, as I have a part-time job in childcare at a gym. I am constantly having to work with children who have varying levels of attachment issues when their parents leave. I find that most children I work with have secure attachment with their parents ("an infant reacts to a mom's departure by becoming upset and greets her return with joy" Lilienfeld, pg. 386). However, with progression of time, children eventually become comfortable and familiar with the childcare environment and gradually do not struggle, cry, or become emotionally upset. I have found a pattern in most children that when they are brought when they are very young (anywhere from 2 months to 18 months), there is usually not a struggle when their parents leave. However, around the age of 2, children who usually have been comfortable in the childcare environment realize that their parents are leaving and struggle. This occurrence usually causes a child to come less frequently. I would be curious to know what psychologists have found out about this and if there is any more information on such an idea.